A typical electrical discharge machining apparatus comprises means for holding a workpiece constituting a first electrode, a tool electrode spacedly juxtaposed with the first electrode across a machining gap, means for applying discharge-voltage pulses across the two electrodes to effect breakdown of the gap and produce an electrical discharge of a transient or short duration so that workpiece material is electrically eroded, a servosystem or the like for maintaining the gap width at the desired level, and means for supplying a dielectric liquid to the gap to sweep the removed detritus and discharge products therefrom and restore a nonconductive state at the gap.
The aforementioned applications and patents relate to various aspects of the control of such apparatus and detail several components of the basic installation.
It has been proposed heretofore to provide voltage pulses of a predetermined duration and interval across the tool electrode and the workpiece to produce respective electric discharges with the discharge current passing for the duration of the applied pulse. High-amplitude voltages may be applied to promote breakdown of the dielectric in the gap. Each time a voltage pulse is supplied, the discharge current flows and a "discharge pulse" is said to be created at the gap, thereby removing material from the workpiece.
However, gap conditions may not be satisfactory to sustain a machining-type discharge and thus various defects may develop in the discharge which is created. For example, if the normally transient or short-duration discharge does not extinguish, a continuous or stationary arc-type discharge may result, thereby producing local overheating without material removal and detrimentally affecting the workpiece, the machining electrode or both. Under certain circumstances no discharge can be created at the gap when the voltage pulse is applied. Both of these latter abnormal conditions, characterized herein as "unsatisfactory" or abnormal discharges are to be contrasted with satisfactory or effective discharges in which the discharge current commences with triggering of the breakdown of the gap, terminates with extinction of the voltage pulse and is effective for the duration of the discharge to remove workpiece material.
It has been found that the normal, satisfactory or good discharge is characterized by an evaluatable range of discharge voltage, discharge current, gap resistance or gap impedance (measured between the machining electrode and the workpiece), the particular range being dependent upon the nature of the tool electrode and the workpiece material, the nature of the liquid dielectric, and the machining condition established to bring about particular machining modes.
The normal, satisfactory or good discharge is also characterized by the presence of a high-frequency component of about 10 megacycles (MHz) which is found superimposed upon the voltage and current waveform.
When the discharge is of the stationary or continuous-arc type, representing abnormal, bad or unsatisfactory discharges, the discharge voltage has a value below the minimum of its range, the discharge current has a value exceeding the maximum of its range and the high-frequency component is diminished or nonexistant in the discharge waveform. It is thus possible to distinguish between normal and abnormal pulses in each pulse cycle by determining whether the discharge voltage or the discharge current is in a predetermined range and whether the discharge contains the high-frequency component mentioned earlier.
While typical normal discharges and typical abnormal discharges can readily be detected in this manner, it has also been found that discharges take place under extremely complex and varying conditions so that there may be formed a semiabnormal discharge which is distinguishable from a typical abnormal discharge but is detrimental in EDM processes. The existence of this semiabnormal discharge has made it difficult, if not impossible, heretofore to provide complete adaptive control of the machining process or system.